A synchronization-type linear motor that uses a magnet as a field system is known. This linear motor includes a movable element having a magnet, and a stator (stationary element) having a core and a coil wound around the core. The core has a salient pole that faces the magnet of the movable element. A coil is wound around the salient pole. A typical coil is a three-phase coil. When a three-phase alternating current, with a 120-degree difference between each two phases, flows through the three-phase coil, an interaction of a magnetic flux generated in the magnet and a magnetic flux generated in the core causes the movable element to move relative to the stator in a direction.
In a conventional linear motor, the core of the stator faces the magnet of the movable element, and therefore there is a problem, i.e., a magnetically attractive force that is greater than a propulsive force acts between the core and the magnet. In order to reduce the magnetically attractive force, Patent Literature Document 1 discloses a so-called tunnel-type linear motor that has a movable element moving in a space closed by a core of a stator. The feature of this linear motor resides in the structure of the core. Specifically, the core of the stator has a first facing portion, with upper magnetic pole teeth and lower magnetic pole teeth facing each other, and a second facing portion, with upper magnetic pole teeth and lower magnetic pole teeth facing each other. The first facing portion and the second facing portion are alternately formed such that the polarity of the first facing portion is opposite the polarity of the second facing portion. When a current flows through the coil, a magnetic flux that alternates up and down is generated in the space sandwiched between the upper magnetic pole teeth and the lower magnetic pole teeth of the first and second facing portions. The movable element moves through the magnetic flux tunnel.
As another invention to reduce the attractive force that acts on the core of the stator and the magnet of the movable element, Patent Literature Document 2 discloses a linear motor that has two stators above and below the movable element (see Patent Literature Document 2, FIG. 8). The movable element has a yoke, which is made from a magnetic material, and a magnet disposed on the yoke. Each of the two stators has a core and a coil wound around a salient pole of the core. Because the two stators are disposed above and below the movable element respectively, the attractive force acting between the upper stator and the movable element can be counterbalanced by an attractive force acting between the lower stator and the movable element.